1. Field of the Invention
The present invention relates to a steering sensor unit which is mounted on a steering shaft of an automobile or the like for detecting information about rotation of a steering wheel and establishing electrical connection between both electric systems provided on the steering wheel and a vehicle body.
2. Description of the Related Art
Heretofore, the so-called steering sensor unit has been proposed which has a one-piece structure comprising a steering sensor for detecting information about rotation of a steering wheel and a rotary connector for electrically connecting an electric system on the steering wheel and an electric system on a vehicle body to each other.
A steering sensor detects the angle and the direction by and in which a steering wheel is turned, and provides information for use in, e.g., damping force control of suspensions, shift position control of an automatic transmission, and steering control of rear wheels in four-wheel steerable (4WS) vehicles. Generally, the steering sensor comprises a code plate attached to a rotor rotated by the steering wheel, and a pair of light emitting and receiving devices disposed on an inner surface of a case (or a casing) which serves as a stator. On the other hand, a rotary connector is capable of establishing and maintaining electrical connection between an electric system provided on the steering wheel as a rotatable member and an electric system provided on a vehicle body as a stationary member. Rotary connectors are mainly divided into two types. In one type, a flexible electric cable or an optical fiber cable (hereinafter referred to generically as a cable) is disposed in a loosely coiled state between a rotor rotated by the steering wheel and a case serving as a stator, the cable having one end fixedly connected to the rotor and the other end fixedly connected to the stator. Electrical connection between the electric systems on both sides can be maintained through reeling-in and -out of the loosely coiled cable. In the other type, one or more isolated electrical paths are formed on a rotor rotated by the steering wheel, and a brush provided on a case serving as a stator is held in slide contact with the electrical paths. Electrical connection between the electric systems on both sides can be maintained through electrical contacts between the electrical paths and the brush.
To reduce the size and simplify the structure, a conventional steering sensor unit is constructed such that a code plate as a component of a steering sensor is held between a rotor of the steering sensor and a rotor of a rotary connector, the two rotors and the code plate being screwed together into a one-piece unit. Looking at circumstances until now, steering sensor units have been developed from rotary connectors, and rotary connectors have been mainly employed to activate air bag devices and disposed near steering wheels so far. For this reason, when mounting the conventional steering sensor unit on a steering shaft, it has been customary that the rotary connector is disposed in a position adjacent to the steering wheel, the steering wheel and the rotor of the rotary connector are coupled to each other by a joint portion formed on the rotor, and the steering sensor is disposed below the rotary connector. Further, in the conventional steering sensor unit, input/output terminals of drive circuits for a light emitting device and a light receiving device are disposed on an outer circumferential surface of a case covering the code plate.
Meanwhile, steering sensor units for use with steering wheels of automobiles to detect a steering angle, for example, are particularly required to have high accuracy in detection of the steering angle for realizing various control processes with high accuracy. Those steering sensor units are also required to be small in size for facilitating design and layout of components around the driver's seat. Further, those steering sensor units are required to have good operability so that smooth operation of the steering wheel will not be impaired.
However, the conventional steering sensor unit has problems as follows. The steering sensor unit is mounted on the steering shaft such that the rotary connector is disposed in a position adjacent to the steering wheel and the steering sensor is disposed below the rotary connector. Accordingly, for example, when the steering wheel is abruptly turned, a torsion due to elastic deformation of the parts tends to occur in a system leading from the joint portion to the rotor of the steering sensor through the rotor of the rotary connector. This results in that a delay in rotation of the rotor of the steering sensor with respect to the rotation of the steering wheel is increased and hence the steering sensor cannot output an accurate pulse signal corresponding to the rotation of the steering wheel. Also, when the operating (rotating) direction of the steering wheel is changed, there occurs the so-called hysteresis that an output signal of the steering sensor has different values even at the same rotational position between when the steering wheel is turned clockwise and when the steering wheel is turned counterclockwise. This also makes it difficult to detect the rotational angle of the steering wheel with high accuracy.
Further, the use of the rotary connector as a device for electrically connecting an electric system on the steering wheel and an electric system on the vehicle body to each other raises another problem below. Specifically, in any type of the rotary connector wherein electrical connection is maintained through reeling-in and -out of a loosely coiled cable, or by holding a brush disposed on a case serving as a stator in slide contact with electrical paths disposed on a rotor, a clearance must be kept between the rotor and the case serving as the stator for ensuring smooth rotation of the rotor. Therefore, an eccentricity tends to occur between the rotor and the case when the rotor of the rotary connector is coupled to the steering wheel through the joint portion provided on the rotor of the rotary connector and the case of the rotary connector is attached to a predetermined mounting portion set on the vehicle body. Because the rotor of the steering sensor and the rotor of the rotary connector are joined together and an integral case is used to cover both the steering sensor and the rotary connector, an eccentricity occurred between the rotor of the rotary connector and the case would give rise to an eccentricity between the rotor of the steering sensor and the case as well. In this respect, with the steering sensor disposed below the rotary connector, the amount of eccentricity between the rotor of the steering sensor and the case would be larger than the amount of eccentricity between the rotor of the rotary connector and the case. Thus, the accuracy in detecting the rotational angle of the steering wheel is deteriorated correspondingly.
Also, even if the rotor of the steering sensor is coupled to the steering wheel and the rotary connector is disposed below the steering sensor, an eccentricity occurred between the rotor of the rotary connector and the case would deviate the rotor of the steering sensor as well and hence deteriorate the accuracy in detecting the rotational angle of the steering wheel similarly to the above.
Further, in the conventional steering sensor unit, because an external output connector of the steering sensor is disposed on the outer circumferential surface of the case covering the code plate, the steering sensor has a large size in the radial direction. This leads to disadvantages of restricting the degree of freedom in design and layout of components around the driver's seat and reducing the efficiency of wiring and other incidental work.